Electrochromic particles

ABSTRACT

The invention concerns discrete electrochromic particles comprising conducting, semiconducting or insulating nanoparticles having one or more electrochromic compounds adsorbed on the surface thereof. These particles may be deposited on an electrode in a single process step at a relatively low temperature, thereby allowing the use of heat-sensitive materials such as plastics as flexible substrates for the electrode.

This invention relates to electrochromic particles. In particular, itrelates to electrochromic particles suitable for use in electrochromicdevices.

Electrochromic devices comprising electrodes based on nanostructuredconducting or semiconducting films having surface-adsorbedelectrochromic compounds are known from inter alia WO-A-98/35267 andWO-A-01/27690. Such electrodes are prepared by applying thenanostructured conducting or semiconducting film to a conductingsubstrate and annealing at high temperatures, followed by chemisorptionof the electrochromic compounds on the surface of the nanoparticles inthe film. This is a time-consuming procedure and also limits theelectrode substrate materials to high temperature-resistant materialssuch as glass or ceramics. It would be desirable to provide conducting,semiconducting or insulating nanoparticles having electrochromiccompounds adsorbed on their surface before their application to asubstrate, thereby avoiding the disadvantages of the prior art.

According to the present invention there are provided discreteelectrochromic particles comprising conducting, semiconducting orinsulating nanoparticles having one or more electrochromic compoundsadsorbed on the surface thereof.

The invention also provides a process for the preparation of theelectrochromic particles of the invention, electrodes comprising saidparticles and their use in the manufacture of electrodes forelectrochromic devices. The electrodes of the invention may be rigid orflexible depending on the choice of substrate material.

As used herein, the term “electrochromic compounds” or“(electro)chromophores” is intended to refer to compounds which changecolour on the application of an electrical potential thereto, butexcluding polymers and inorganic compounds.

As used herein, the term “nanoparticles” is intended to refer todiscrete and dispersible particles having an average particle size of upto 80 nm, preferably up to 50 nm, and more preferably up to 30 nm.

As used herein, the term “conducting nanoparticles” is intended to referto nanoparticles having no electronic bandgap; the term “semiconductingnanoparticles” is intended to refer to nanoparticles having a bandgapless than or equal to 5 electron Volts; and the term “insulatingnanoparticles” is intended to refer to nanoparticles having a bandgapgreater than 5 electron Volts.

The electrochromic particles of the invention may be in the form of asolid or suspended in a solvent.

The electrochromic compound(s) is/are preferably adsorbed on the surfaceof the nanoparticles so that there is up to 100% monolayer coverage ofthe nanoparticles and at least 1% monolayer coverage.

Conducting or semiconducting nanoparticles are preferred. Preferrednanoparticles are selected from doped or undoped oxides of the followingmetals: titanium, zirconium, hafnium, chromium, molybdenum, indium, tin,tungsten, vanadium, niobium, tantalum, silver, zinc, cerium, strontium,iron (2⁺ and 3⁺) or nickel, or a perovskite thereof, preferably TiO₂,WO₃, SnO₂, MoO₃, In₂O₃/SnO₂ or ZnO. Suitable dopants include F, Cl, Sb,P, As, B, Al, In, Ga, Si, Sn, Ti, Ge, Zr, Li and Hf.

Insulating nanoparticles which can be used in the present inventioninclude oxides of silicon, aluminium, zirconium, barium, magnesium andsodium.

The electrochromic compounds adsorbed on the surface of thenanoparticles may be the same or different and are conveniently of then-type or p-type. Preferred electrochromic compounds for use in thisinvention are disclosed in WO-A-98/35267, WO-A-01/27690, WO-A-03/001288and a copending PCT Patent Application entitled “ElectrochromicCompounds”, filed on even date by the Applicant (NTera Limited).Particularly preferred n-type compounds includebis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride,1-phosphonoethyl-1′-(2,4,6-trimethylphenyl)-4,4′-bipyridinium dibisimideand 1-phosphonoethyl-1′-(4-styryl)-4,4′-bipyridinium diperchlorate.

Particularly preferred p-type compounds include:β-(10-phenothiazyl)propoxy phosphonic acid;β-(10-phenothiazyl)propyl-phosphonic acid; β-(10-phenothiazyl)propionatephosphonic acid; β-(10-phenoxazyl)propionate phosphonic acid; and(1-ferrocenyl)imido-benzylmethyl phosphonic acid.

The electrochromic compounds used in the present invention may alsoinclude reactive groups that can be activated to form a chemical bondbetween adjacent electrochromic compounds on the same particle or onadjacent electrochromic particles, hereinafter referred to ascrosslinking groups. These groups are conveniently positioned on theelectrochromic molecules at the opposite end of the surface attachmentgroup. Alternatively, the crosslinking groups may be attached to thenanoparticles via, for example, an alkyl group which in turn is linkedto a nanoparticle surface attachment group. Because each particle maycontain many of these reactive groups on its exterior surface, theactivation of these groups leads to a three-dimensional crosslinking ofthe particles. The activation may be initiated by thermal, ionic,reductive, oxidative, radical, photochemical or electrochemical means.Suitable reactive groups include vinyl, styryl, (meth)acrylates,epoxies, silanes, amines, alcohols, carboxylic acids and carboxylic acidhalides. In some cases, activation may occur by reaction with anadditional chemical entity, e.g. a bridging molecule such as adi-carboxylic acid, di-amine or di-alcohol.

The following table illustrates the above:

Crosslinking Schemes Crosslinking Activation Group Code General formulaschemes Vinyl R1

A1, A2, A3, A3 +A4, A5. Styryl R2

A1, A2, A3, A3 +A4, A5. Acrylate R3

A1, A2, A3, A3 +A4, A5. Epoxy R4

A7, A8. Alcohol R5

A6 + A1. Amine R6

A6 + A1. Carboxylic acid R7

A7 + A1, A8 +A1.

Activation Schemes Formula (where Activator Code applicable) Heat A1Ionic A2 Examples: Butyl lithium, Aluminium trichloride. Light A3Typically, UV light. Photoinitiator A4 Example: azobisisobutyronitrile(AIBN) Electrochemical A5 reduction or oxidation Di-carboxylic acid A6

Di-alcohol A7

Di-amine A8

The crosslinking group is attached to the chromophore moiety via thebond depicted in bold. This crosslinking group can be activated viadifferent schemes, some of which require the addition of initiators(i.e. A2, A4, A6, A7, A8) which will be included in the formulation tobe printed, some of which require only heat or light as an initiator.The preferred schemes involve an activation process step after printing.This allows better control of when the printed film can be crosslinked.The preferred methods therefore include exposure to heat, light or anelectrochemical potential (A1, A3 and A5), that may or may not befacilitated by the presence of additional chemical initiators in theformulation and film.

Crosslinking of the electrochromic particles enhances the mechanicalstrength of the resulting film. Crosslinking dispenses with the need fora polymeric binder and renders the electrochromic particles particularlysuitable for ink-jet printing.

The electrochromic particles of the present invention may additionallycomprise on the surface of the nanoparticles one or more compounds whichprevent or inhibit aggregation of the electrochromic compounds adsorbedon the nanoparticles. Suitable aggregation-inhibiting compounds includealkane phosphonates and cationic pyridinium carrying one or moreanchoring functional groups, such as phosphonoethylpyridiniumperchlorate.

The electrochromic particles of the invention may be prepared by mixingthe conducting, semiconducting or insulating nanoparticles and one ormore electrochromic compounds in a solvent and optionally isolating theresulting electrochromic particles.

The nanoparticles may be suspended in a solvent prior to mixing with theelectrochromic compound(s) in a solvent. In the latter event, thenanoparticle solvent and the electrochromic compound solvent arepreferably the same. The mixing is typically carried out at atemperature of approximately 25° C. for a period of from approximately30 minutes to 2 hours. The resulting electrochromic particles may beisolated by any suitable means, such as, for example, by centrifugation,and dried at a temperature in the range of from approximately 50° C. to90° C. for approximately 6 to 30 hours.

Solvents suitable for suspending the nanoparticles and electrochromiccompounds, and dispersing the dried electrochromic particles includediethyl ether, 1,1,1-trichloroethane, amyl acetate, carbontetrachloride, xylene, ethyl acetate, toluene, tetrahydrofuran,N-methylpyrrolidone, benzene, chloroform, trichloroethylene, methylethyl ketone, acetone, diacetone alcohol, ethylene dichloride, methylenechloride, pyridine, morpholine, dimethylformamide, dimethyl sulphoxide,methanol, ethanol, n-propanol, n-butanol, propylene glycol, ethyleneglycol, glycerol, and water. Preferred solvents include ethanol,N-methylpyrrolidone and water.

To form an electrode, the dried electrochromic particles may bedispersed in a solvent, such as N-methylpyrrolidone or polyvinyldifluoride, so as to form a paste which is then applied to a suitablesubstrate by, for example, stencil-coating or ink-jet printing orscreen-printing. The paste on the substrate may be dried at atemperature in the range of from about 50° C. to 200° C., preferablyabout 80° C. to 150° C. The substrate may be formed from e.g. glass,ceramic, metal or plastic, optionally coated with a layer of conductingmaterial, such as tin oxide doped with fluorine or antimony.

In cases where one or more of the electrochromic compounds present onthe electrochromic particles, include one or more crosslinking groups, acrosslinking initiator may be added to the dispersion of driedelectrochromic particles.

An electrochromic device may be formed by preparing a counter electrode,sealing the counter electrode to the electrode comprising theelectrochromic particles of the invention and incorporating anion-conducting medium.

The electrochromic particles of the invention have the followingadvantages:

-   -   1. They may be deposited on an electrode in a single process        step at a relatively low temperature, thereby allowing the use        of heat-sensitive materials such as plastics as flexible        substrates for the electrode;    -   2. They allow greater control of the pixel size and resolution        of the final image, especially where small pixel size and more        than one coloured pixel are required on the same electrode or        image;    -   3. The colour of the electrochromic particles can be controlled        by changing the nature and relative amounts of the chromophores.        For example, (i) electrochromic particles with different        chromophores can be mixed homogeneously and the resulting film        will display the corresponding mixed colour, and (ii) different        electrochromic particles with different chromophores can be        deposited side by side on a substrate thereby providing the        possibility of increasing the information content of the        display. Such pixels composed of electrochromic particles can be        independently addressed, so that a high resolution multicolour        (polychromic) display device can be achieved.

The invention is illustrated in the following Examples.

EXAMPLE 1

Electrochromic particles were prepared by dissolving 0.1Mbis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride (1.118 g) as thechromophore and 0.01M lithium perchlorate (0.0266 g) in 25 ml of water.5 g of TiO₂ powder (30 nm average particle size) were added to thechromophore solution. The mixture was stirred at 25° C. for 1 hour andsubsequently centrifuged at 5000 rpm to separate the electrochromicparticles. The electrochromic particles were redispersed in ethanol todissolve any non-chemisorbed chromophore molecules and separated againby centrifugation at 5000 rpm from the rinse solution. The washed solidswere subsequently dried at 70° C. for 24 hours.

For the preparation of a cathode, 4 g of the dried electrochromicparticles were dispersed in 6 g of N-methyl pyrrolidone to form a paste.This paste was stencil-coated on to a fluorine-doped tin oxide (FTO)glass substrate which was dried using the following thermal cycleimmediately after printing of the paste:

From room temperature (25° C.) to approximately 100° C. over a 15 minuteperiod;

At 100° C. for 30 minutes;

Cool down to room temperature (25° C.) over a period of 30 minutes.

The resulting cathode was then rinsed in ethanol to remove anynon-chemisorbed chromophore molecules.

EXAMPLE 2

Electrochromic particles were prepared by dissolving 500 mg of mesidinebisimide (i.e.1-phosphonoethyl-1′-(2,4,6,trimethylphenyl)-4,4′-bipyridiniumdibisimide) in 100 ml of methanol and adding this solution over a periodof 20 minutes to 200 ml of methanol containing 2.5 g of suspended TiO₂nanoparticles. Mesidine bisimide is an electrochromic compound which isdisclosed and claimed in a copending European Patent Applicationentitled “Electrochromic Compounds”, filed on even date by the Applicant(NTera Limited). When the addition was complete, the mixture was stirredat 25° C. for 1 hour and subsequently centrifuged at 5000 rpm toseparate the electrochromic particles. The electrochromic particles wereredispersed in ethanol to dissolve any non-chemisorbed chromophoremolecules and separated again by centrifugation at 5000 rpm from therinse solution. The washed solids were subsequently dried at 70° C. for24 hours.

For the preparation of a cathode, 4 g of the dried electrochromicparticles were dispersed in 6 g of N-methyl pyrrolidone to form a paste.The paste was stencil-coated onto a fluorine doped tin oxide (FTO) glasssubstrate which was dried using the following thermal cycle immediatelyafter printing of the paste:

From room temperature (25° C.) to 60° C. over 15 minutes; At 60° C. for30 minutes;

Cool down to room temperature (25° C.) over a period of 30 minutes.

The resulting cathode was then rinsed in ethanol to remove anynon-chemisorbed chromophore molecules.

EXAMPLE 3

Electrochromic particles were prepared by dissolving 500 mg of1-phosphonoethyl-1′-(4-styryl)-4,4′-bipyridinium diperchlorate in 200 mlof methanol containing 2.5 g of suspended TiO₂ nanoparticles.1-Phosphonoethyl-1′-(4-styryl)-4,4′-bipyridinium diperchlorate isdisclosed in WO-A-03/001288. The mixture was stirred at 25° C. for 1hour and subsequently centrifuged at 5000 rpm to separate theelectrochromic particles. The electrochromic particles were redispersedin ethanol to dissolve any non-chemisorbed chromophore molecules andseparated again by centrifugation at 5000 rpm from the rinse solution.The washed solids were subsequently dried at 70° C. for 24 hours.

For the preparation of a cathode, 4 g of the dried electrochromicparticles were dispersed in 6 g of N-methyl pyrrolidone containing 300mg azobisisobutyronitrile (AIBN) as photoinitiator to form a printingpaste. The paste was stencil-coated onto an indium tin oxide (ITO) glasssubstrate. Immediately after coating, the substrate was dried, and thefilm was crosslinked at 80° C. for 60 minutes.

EXAMPLE 4

A cathode (40 mm×40 mm) was prepared according to the procedure ofExample 1.

For the preparation of a counter electrode, a glass substrate coatedwith fluorine doped tin oxide (FTO) as transparent conducting oxide (50mm×50 mm) was coated with antimony doped tin oxide (ATO) byscreen-printing and heated at 60° C. for 20-30 minutes. A whitereflector paste comprising white pigment particles of rutile titania wascoated by screen-printing over the ATO layer and the double layer wasallowed to sinter at 450° C. for 45 minutes.

The cathode was sealed to the counter electrode using an epoxy gasketseal. The resulting electrochromic device was backfilled with a 0.2Melectrolyte solution of lithium perchlorate in gamma butyrolactone undervacuum and cured under UV light. Application of 1.3V across thiselectrochromic device resulted in colouration of the device.

In the accompanying drawings:

FIG. 1 shows the UV-visible reflectance spectra of the electrochromicparticles prepared in Example 1;

FIG. 2 is a cyclic voltammogram of the electrochromic device of Example4, with a scan rate of 10 mV/sec. The onset voltage for devicecolouration is approximately −0.8 Volt, which is very advantageous interms of device energy consumption;

FIG. 3 shows the chronocoulometric behaviour (charge versus time) of thedevice of Example 4 when a −1.3 Volt pulse is applied to the device. Theamount of charge corresponding to full colouration is approximately 4mC/cm². Combined with the low onset voltage, this charge consumptionyields a low power consumption for the device comprising these films;and

FIG. 4 shows the reflectance of the device of Example 4 in the bleachedstate (short circuit, denoted by a solid line

) and coloured state (immediately after applying the colouration voltage(−1.3V) pulse, denoted by a dashed line

), and after 15 minutes held at open circuit, concurrently (denoted by abroken line

). A reflectance as high as 45.62% is obtained in the bleached state andas low as 7.74% in the coloured state resulting in a contrast ratio of5.89. These results compare very favourably with those obtained usingdevices of the type disclosed in WO-A-98/35267 and WO-A-01/27690.

1. Discrete electrochromic particles comprising conducting,semiconducting or insulating nanoparticles having one or moreelectrochromic compounds adsorbed on the surface thereof. 2.Electrochromic particles according to claim 1, which are in the form ofa solid or a suspension.
 3. Electrochromic particles according to claim1, wherein the nanoparticles have an average particle size of up to 50nm, preferably up to 30 nm.
 4. Electrochromic particles according toclaim 1, wherein the nanoparticles are conducting or semiconducting. 5.Electrochromic particles according to claim 1, wherein the nanoparticlesare selected from doped or undoped oxides of the following metals:titanium, zirconium, hafnium, chromium, molybdenum, indium, tin,tungsten, vanadium, niobium, tantalum, silver, zinc, cerium, strontium,iron (2⁺ and 3⁺) or nickel, or a perovskite thereof, preferably TiO₂,WO₃, SnO₂, MoO₃, In₂O₃/SnO₂ or ZnO.
 6. Electrochromic particlesaccording to claim 5, wherein the metal oxides are doped by F, Cl, Sb,P, As, B, Al, In, Ga, Si, Sn, Ti, Ge, Zr, Li or Hf.
 7. Electrochromicparticles according to claim 1, wherein the electrochromic compounds areof the n-type or p-type.
 8. Electrochromic particles according to claim7, wherein the electrochromic compounds are of the n-type and areselected from bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride,1-phosphonoethyl-1′-(2,4,6-trimethylphenyl)-4,4′-bipyridinium dibisimideand 1-phosphonoethyl-1′-(4-styryl)-4,4′-bipyridinium diperchlorate. 9.Electrochromic particles according to claim 7, wherein theelectrochromic compounds are of the p-type and are selected fromβ-(10-phenothiazyl)propoxy phosphonic acid;β-(10-phenothiazyl)propyl-phosphonic acid; β-(10-phenothiazyl)propionatephosphonic acid; β-(10-phenoxazyl)propionate phosphonic acid; and(1-ferrocenyl)imido-benzylmethyl phosphonic acid.
 10. Electrochromicparticles according to claim 1, wherein the or each electrochromiccompound includes one or more crosslinking groups.
 11. Electrochromicparticles according to claim 10, wherein the crosslinking groups areselected from vinyl, styryl, (meth)acrylates, epoxies, silanes, amines,alcohols, carboxylic acids and carboxylic acid halides. 12.Electrochromic particles according to claim 1, wherein the nanoparticlesadditionally comprise on their surface one or more compounds whichprevent or inhibit aggregation of the electrochromic compounds adsorbedon the nanoparticles.
 13. Electrochromic particles according to claim12, wherein the aggregation-inhibiting compounds are selected fromalkane phosphonates and cationic pyridinium carrying one or moreanchoring functional groups.
 14. A process for preparing discreetelectrochromic particles comprising conducting, semiconducting orinsulating nanoparticles having one or more electrochromic compoundsadsorbed on the surface thereof, comprising mixing the conducting,semiconducting or insulating nanoparticles and one or moreelectrochromic compounds in a solvent and optionally isolating theresulting electrochromic particles.
 15. A process according to claim 14,wherein the nanoparticles are isolated and suspended in a solvent priorto mixing with the electrochromic compound(s) in a solvent.
 16. Aprocess according to claim 15, wherein the solvents used to suspend thenanoparticles and dissolve the electrochromic compound(s) are the same.17. A process according to claim 14, wherein the solvent is ethanol,N-methyl pyrrolidone or water.
 18. A process according to claim 14further comprising drying the isolated electrochromic particles.
 19. Aprocess according to claim 18 further comprising dispersing the driedelectrochromic particles in a solvent so as to form a paste and applyingsaid paste to a substrate, so as to form a first electrode.
 20. Aprocess according to claim 19, further comprising drying said paste onthe substrate at a temperature in the range of from about 50° C. toabout 200° C., preferably about 80° C. to 150° C.
 21. A processaccording to claim 19, wherein the paste is applied by stencil-coatingor ink-jet printing or screen-printing.
 22. A process according to claim19, wherein the substrate is formed from glass, ceramic, metal orplastic optionally coated with a layer of conducting material, such astin oxide doped with fluorine or antimony.
 23. A process according toclaim 19, wherein one or more electrochromic compounds on theelectrochromic particles include one or more crosslinking groups and thepaste also comprises a crosslinking initiator.
 24. A process accordingto claim 19, further comprising preparing a counter electrode, sealingthe counter electrode to the first electrode and incorporating anion-conducting medium, so as to form an electrochromic device.
 25. Anelectrode or an electrochromic device comprising electrochromicparticles according to claim
 1. 26. Use of electrochromic particlesaccording to claim 1 in the manufacture of an electrode or anelectrochromic device.